1. Field of the Invention
The present invention relates to a thermal printer, in particular, to an improvement in the structures of a thermal printhead unit and a platen roller unit.
2. Description of the Prior Art
A thermal printer is configured to include a thermal printhead to print information on a thermal paper. In order to realize high-quality printing, it is essential to tightly place a paper into contact with an exothermic element array of the thermal printhead. A platen roller is provided to press the paper onto the exothermic element array.
Meanwhile, various kinds of papers are available for the thermal printer, and it is well known that the level of contact with the exothermic element array differs depending on the thickness of a paper.
For example, with use of a thin paper, it is possible to bring the paper in close contact with an appropriate portion of the exothermic element array for proper printing. However, with use of a thick paper, it may not be able to bring the paper in close contact with an appropriate portion of the exothermic element array owing to a large rigidity of the thick paper in addition to a displacement of the thermal printhead.
In view of solving the above problem, Japanese Patent Application Publication No. 2006-315285 and No. 2009-101524 disclose a thermal printer with a thermal printhead whose position is changeable in accordance with the thickness of a paper in use, to be able to adjust a positional relation between the exothermic element array and the paper and properly place the paper in close contact with the exothermic element array even with use of a thick paper.
Specifically, in the thermal printer of the above documents the thermal printhead is configured to be movable relative to the platen roller between two different positions in a circumference direction of the platen roller. With such a configuration, it aims to constantly maintain the position of the exothermic element array relative to the rotary axis of the platen roller irrespective of the thickness of a paper by moving the thermal printhead forward/backward in the paper forwarding direction according to the thickness of the paper.
However, in reality, the position of the exothermic element array tightly contacting with the paper shifts depending on the thickness of the paper due to rigidity of the paper which increases in accordance with the thickness.
Therefore, a problem arises with the thermal printer disclosed in the above documents that the exothermic element array cannot be brought into a close contact with a paper in a large thickness.
Furthermore, the thermal printhead and the platen roller are preferably configured to be manually attachable/detachable easily for replacement without use of any tool, in order to reduce time and labor taken for detaching/attaching them.
Moreover, generally, a body of the thermal printer is provided with a cover element to open/close for the purpose of replenishing or replacing a paper. It is configured that with the cover element open, a paper can be set in a paper container in the body.
The thermal printer is therefore preferably configured to include the platen roller in the body and the thermal printhead on the cover element and to easily complete paper setting to place a top end of the paper between the platen roller and the thermal printhead by simply closing the cover element.